Contact lenses are ophthalmic lenses worn on the anterior cornea that are widely used for correcting many different types of vision deficiencies. These include defects such as near-sightedness (myopia) and far-sightedness (hypermetropia), astigmatism, and defects in near-range vision usually associated with aging (presbyopia). A posterior surface of the contact lens fits against the cornea and an opposite anterior surface has an optical zone that refracts light to correct vision.
Astigmatism occurs when the refractive error in an eye is meridian-dependent. This is usually due to one or more refractive surfaces, most commonly the anterior cornea, having a toroidal shape. It may also be due to one or more surfaces being transversely displaced or tilted. Astigmatism is usually regular, which means that the principal (maximum and minimum power) meridians are perpendicular to each other. People with astigmatism have blurred vision at all distances, although the problem may be worse in their distance or near vision, depending on the type of astigmatism. These people may complain of sore eyes and headaches associated with demanding visual tasks.
Astigmatism can be corrected with contact lenses, usually having one spherical surface and one toroidal (sphero-cylindrical) surface. Due to limitations of traditional diagnostic devices such as phoropters, the astigmatic refractive errors traditionally corrected are second-order surface or wavefront functions, so conventional astigmatism correction with contact lenses is relatively invariant of registration to the eye's line-of-sight. However, the correction is dependent on lens rotation on the eye. So contact lenses with an astigmatism correction include an orientation feature to maintain a predetermined orientation on the eye.
The current industry standard is to provide contact lenses with astigmatic corrections beginning at −0.75 Diopter. Typically, one or two additional cylinder powers, in 0.5 or 0.75 Diopter steps, are offered. These corrections are included in lens series having spherical power corrections (for myopia or hypermetropia) in 0.25 Diopter increments. Typical parameter ranges include axes parameters in no finer than 10-degree increments. Patients are prescribed with lenses having whichever of these astigmatic corrections most closely matches the needs of each of their eyes. Typically, an astigmatic error of less than 0.75 Diopter is not corrected with contact lenses. More precise astigmatism correction has not been deemed practical or necessary.
In addition to second-order astigmatism, human eyes usually have a number of other aberrations in refractive surfaces, though mild amounts of such refractive errors are often asymptomatic or cause only generally tolerable vision deficiencies. Recent improvements in technology, such as ophthalmic wavefront sensors, have provided higher resolution than the standard sphero-cylindrical averaged refractive error. Zernike polynomials are typically used to describe refractive errors for on-axis optical systems. Zernike basis sets can accurately describe a map of the full refractive error. Until recently, the only Zernike mode, other than sphere, that was corrected with contact lenses was second-order astigmatism. Recent efforts are believed to have been directed to attempting to provide optimal vision by customizing contact lenses using a Zernike basis set to simultaneously cancel all of the measurable aberrations in a particular eye. But correcting for higher-order aberrations requires registration of the contact lens to the eye's line-of-sight. In addition, the resulting contact lenses have extremely complex surfaces, which are very costly and difficult to precisely manufacture.
Thus it can be seen that needs exist for improvements to ophthalmic lenses to correct for aberrations to provide premium vision. It is to such improvements that the present invention is primarily directed.